To curb the continuing obesity epidemic, `upstream' prevention efforts are desperately needed. To inform such prevention efforts, increased scientific knowledge of the following is required: 1) the mechanisms through which physical activity (PA) in pregnancy, a modifiable lifestyle factor, may impact body size and composition in infancy, and 2) improved methods for evaluating health system based lifestyle interventions to improve maternal gestational weight gain (GWG) and infant birthweight. This K01 will provide Dr. Samantha F. Ehrlich Ph.D., with the expertise needed to achieve her career goals of reducing obesity and diabetes in women and children by promoting and ultimately increasing participation in PA at the population level, particularly through interventions implemented at the health system level. The central hypothesis of this proposal is that increasing PA during pregnancy may mitigate the development of obesity in both the mother (i.e., through appropriate GWG) and child (i.e., through appropriate birthweight and reduced body fat at birth, and developmental programming for appropriate weight/fat mass gains in infancy). The specific aims are as follows. Aim 1: To estimate the effects of PA during pregnancy on infant growth from birth to 1 year of age using novel methods that permit causal inference to be drawn from observational data. Aim 2: To estimate the effects of PA regime during pregnancy (i.e., objectively measured PA in early and late pregnancy) on infant body composition (i.e., percent body fat) from birth to 1 year of age using novel methods that permit causal inference to be drawn from observational data with time-dependent confounding (i.e., measures of PA, diet, and fat mass in early and late pregnancy). Aim 3: To efficiently evaluate a physician-initiated intervention promoting PA and appropriate weight gain in pregnancy implemented at a single medical center by estimating its effects on maternal GWG and infant birthweight as compared to matched patients from non-intervention medical centers. Aim 3 uses existing historical data to match at the medical center level and existing electronic health record data to match on individual-level variables. This research will expand knowledge of potential targets for real world interventions of `upstream' obesity prevention, and use novel methods for evaluating one such intervention. Dr. Assiamira Ferrara MD, Ph.D., Senior Research Scientist and Chief of the Women's and Children's Health Section at the KPNC Division of Research, will be my Primary Mentor (obesity prevention). Co-mentors with complementary expertise include Drs. David Bassett, Jr. (objectively measured PA), Hollie A. Raynor (nutrition and behavioral aspects of obesity prevention), and Romain Neugebauer (causal inference-based methods of analysis); Dr. Kelley Pettee Gabriel (PA epidemiology) is a Consultant. The research, new training and professional developmental afforded by this award will ensure Dr. Ehrlich's successful transition to research independence in the area of PA during pregnancy for intergenerational obesity and diabetes prevention.